The ultraviolet spectrum of the normal giant elliptical NGC 4472.

Abstract

Low resolution IUE spectra of NGC 4472 covering the wavelength region 1100-3300 Å have been obtained. Strong emission lines have probably been found in the short wavelength region. Such emission lines have also been found in M 31, M 32, and in the nucleus of M 87. Most of the lines can be identified with known chromospheric lines, the relative strengths, however do not agree with those found in late type stars and suggest existence of regions with much higher plasma temperatures. Also the total emissivity of the lines is much stronger than that expected from the late type component. We suggest as a possibility that the emission lines originate in a hot, 10⁵, and tenuous, 10^-1 - 10^-2 electron/cm³, corona surrounding NGC 4472. Depending on the temperature structure of such a corona, X-ray emission, due to thermal bremsstrahlung, might be detectable. Apart from the emission lines the galaxy spectrum is in accordance with the composite spectrum expected from published population models. The energy tum up at the shorter wavelength (1300-1800 Å) put rather strong constraints on the type and amount of early type stars present in the nucleus. We cannot decide whether this hot light is due to upper main sequence stars or hot horizontal-branch stars. We find the UV light source to be extended, with a light distribution resembling the visual light distribution. Our main conclusions are supported by the UV spectra of the nuclei of M 87 and NGC 3379. We compare also our results with the published spectra of M 31 and M 32.